Large scale infrared detector arrays are needed for thermal imaging purposes. Thermal imaging has a wide range of applications, from military night vision systems to astrophysics to medical diagnostics. In these applications, individual detectors not only need to be sensitive and fast, but more importantly, their device characteristics have to be uniform. In addition, if a detector array is connected to Charge Coupled Devices (CCD) for signal read-out, the dark current, which is the current flowing through each detector without being exposed to light, has to be small due to the limited charge handling capacity of the CCD. Recently, Levine et al. have disclosed a new multiple quantum well photodetector. This device is discussed in two separate articles, the first of which is "New 10 Micron Infrared Detector Using Intersubband Absorption in Resonant Tunneling GaAlAs Superlattices", Applied Physics Letters, Vol. 50 (1987), pg. 1092-1094. The authors introduce a high speed infrared detector based on intersubband absorption and sequential resonant tunneling in doped GaAs/Al.sub.x Ga.sub.1-x As As quantum well superlattices. They term the device a STAIR detector. In this type of detector, infrared light which is in resonant with the intersubband transition excites electrons from the doped ground state to the excited state where they can tunnel out of the well through the thin top of the barrier. These photogenerated hot electrons then travel a mean free path thereby generating a photocurrent before being captured by the wells. "High Sensitivity Low Dark Current 10 Micron GaAs Quantum Well Infrared Photodetectors,"Applied Physics Letters, Vol. 56 (1990), pg. 851-853, also by Levine et al. contributes no novel features but does extend the range of parameters of the device.
In a related article by Kwong-Kit Choi et al, entitled "Multiple Quantum Well 10 micron GaSa/Al.sub.x Ga.sub.1-x Ga As Infrared Detector With Improved Responsivity," a higher responsivity is achieved in this type of device. This occurs by using thicker and higher Al.sub.x Ga.sub.1-x As layers as superlattice barriers to reduce the dark current. This allowed the detector to be operated at higher biases. Additional background can be obtained from European patent No. 88300096.0 "Infrared Radiation Detector Devices," filed by Clyde G. Bethea et al.
The essential feature of this type of device is that the detector consists of a number of isolated quantum wells. In order to limit the tunneling dark current from the ground state, thick superlattice barriers have to be adopted. However, thick barriers will also limit the photoexcited electrons from tunneling out of the well and forming free electrons. The approach of Levine et al. is to use low barriers so that the excited state is above the barriers. The photoexcited electrons do not need the tunneling process to become free. The disadvantage of this approach is that the low barriers cannot limit the thermally activated dark current which is dominated at high temperatures such as 77.degree. K. In fact, the low barrier detectors, which contain only one confined state, perform worse than a detector with two bound states at 77.degree. K. due to the large thermal dark current at high temperatures.